Method and device for compensating the phase for flat screens

ABSTRACT

The invention relates to a method and a device for matching the phase between the pixel clock of a graphics card and the sampling clock of a flat-panel display with an analog interface in a system comprising flat-panel display, graphics card and computer. Herein the rising edge of a video pulse of a sufficiently bright image spot in the first image column close to the back-porch region is determined. The falling edge of a video pulse at a sufficiently bright image spot in the last image column close to the front-porch region is determined, and the phase is adjusted such that the sampling instant is situated approximately at the midpoint between the rising and falling edges of a video pulse.

RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/DE00/00835, filedon Mar. 17, 2000.

This patent application claims priority of German patent applicationNo.: 199 13 917.2, filed Mar. 26, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and a device for matching the phasebetween the pixel clock of a graphics card and the sampling clock of aflat-panel display with an analog interface in a system comprisingflat-panel display, graphics card and computer.

2. Background Art

Flat-panel displays with an analog interface must be adapted to thegraphics card of the connected computer. If phase or sampling frequencyis incorrectly adjusted, the image appears fuzzy and containsinterferences.

Whereas the values for image location, or in other words right-left andtop-bottom adjustment, and for sampling frequency can be defined aspreadjusted values in the case of standard modes, this is not possiblefor the phase, since the phase depends on the graphics card used andalso on the video circuit.

Prior art flat-panel displays are usually provided with amicroprocessor, which is responsible for general control of theflat-panel display. This microprocessor is configured such that it canalso recognize the video mode adjusted on the computer. If the mode hasalready been adjusted at the factory or by the user, the flat-paneldisplay is operated with the stored adjustments for image location,sampling frequency and phase. On the other hand, if the mode is onewhich has not yet been implemented in the microprocessor of theflat-panel display, standard values are assumed for image location,sampling frequency and phase. These standard values are not satisfactoryin all cases.

The adjustment of the sampling clock and of the phase have a directeffect on image quality. An optimal sampling frequency is achieved whenthe sampling of all pixels, in one line of a video signal, for example,takes place in a stable or characteristic region of these pixels, suchas at the center of each pixel. Data conversion then yields optimalresults. The displayed image does not contain any interferences, and isstable. In other words, the optimal sampling frequency is equal to thepixel frequency. If an incorrect sampling frequency has been adjusted,for example if the sampling clock is too fast compared with the pixelclock, the pixels are sampled at first in the permissible region, or inother words at the midpoint between two edges, but the subsequent pixelsare sampled progressively more toward one edge, until even the regionbetween two pixels is sampled, which obviously leads to unsatisfactoryimage quality. Incorrect sampling values are derived from the region inwhich the pixels are not sampled in an optimal, characteristic region.The image then exhibits strong vertical interference. The number ofregions with vertical interference that are visible on the monitorincreases as the difference between the frequencies of the samplingclock and the pixel clock becomes larger.

Even in the cases in which the sampling clock is identical to the pixelclock, however, the image quality can suffer if the phase has not beenadjusted correctly. The reason is that sampling takes place in a pixelregion that is not ideally suitable for sampling, for example too closeto the leading or trailing edge of a pixel. This problem can be solvedby shifting the phase, or in other words the sampling instant, as thewhole until sampling takes place in a characteristic or permissibleregion of the pixels. If the phase has not been adjusted correctly, theimage quality is impaired by noise signals over the entire monitor.

For this reason the users are usually instructed in the manuals and bynotices on the packing to perform the necessary adjustment of the phasethemselves, but this is unsatisfactory, especially for less experiencedusers.

Flat-panel displays with analog interfaces, in which phase adjustment isautomatically performed, are already known. For such automaticphase-position adjustment, special test patterns with alternating whiteand black image spots are necessary, and the test pattern must bedisplayed by the graphics card. This has the disadvantage that softwaremust be installed and started on the computer, and furthermore that thissoftware must be available for all common operating systems.

From German Patent 3914249 A1 there is known a method for recovery froman input signal generated with an unknown clock, wherein the inputsignal is digitized with a reference clock at different phase positions.The difference between the clock frequency of the input signal and thereference clock is determined from the variation of the phase position(input signal relative to reference clock), and the frequency of thereference clock is corrected accordingly.

A signal-processing method for an analog image signal is described inGerman Patent 19751719 A1. Therein the analog image signal is obtainedfrom a computing unit, in which the signal has been digitally generatedaccording to a graphics standard such as EGA or VGA and then convertedto analog form. The method comprises subjecting the analog image signalto analog-to-digital conversion with a first selected samplingfrequency, after which the sampled image is examined for imageperturbations, in order to determine a corrected sampling frequency.Further measures relate to determination of the optimal sampling phaseand determination of the exact position of the active image relative tothe horizontal or vertical synchronization pulses.

BRIEF SUMMARY OF THE INVENTION

In this regard, the object of the invention is to provide a method and adevice for matching the phase in flat-panel displays, whereby automaticphase adjustment is possible without the use of test patterns.

To achieve this object, an inventive method is characterized in that therising edge of a video pulse of a sufficiently bright image spot isdetermined, in that the falling edge of the video pulse is determined ata sufficiently bright image spot and in that the phase is adjusted suchthat the sampling instant is situated approximately at the midpointbetween the rising and falling edges of a video pulse.

To achieve the said object, an inventive method is further characterizedin that the rising edge of a video pulse of a sufficiently bright imagespot is determined, and in that the phase is adjusted such that thesampling instant is shifted by approximately half the width of an imagespot toward the center of the pixel.

To achieve the said object, an inventive method is further characterizedin that the falling edge of the video pulse is determined at asufficiently bright image spot, and in that the phase is adjusted suchthat the sampling instant is shifted by approximately approximately halfthe width of an image spot toward the center of the pixel.

Whereas the image-location and sampling frequencies can be determinedand correspondingly adjusted relatively simply by an algorithm, thephase position is more difficult to determine. The three said inventivemethods are simple and satisfactory methods for adjusting the phases.

An advantageous embodiment of the inventive method, wherein the imagearea and image spots are arrayed on the flat-panel display in rows andcolumns between a back-porch region and a front-porch region, ischaracterized in that an image spot in the first image column close tothe back-porch region is chosen as the sufficiently bright image spotfor determination of the rising edge and an image spot in the firstimage column close to the front-porch region is chosen as thesufficiently bright image spot for determination of the falling edge.The method can be performed particularly well if the most pronouncedpossible edges are evaluated or if regions or spots disposed next to oneanother have very different brightness. Thus a spot in the first or lastimage column is particularly suitable, since it completely satisfies therequired conditions in combination with the front-porch or back-porchregion respectively, and can be found with relatively little difficulty.

An advantageous embodiment of the inventive method is characterized inthat the brightness of a plurality of image spots of the first or lastimage column is measured, and the image spot with the greatest oradequate brightness in the first or last image column is chosen fordetermination of the rising or falling edge respectively of the videopulse. In this way it is ensured that image spots with sufficientlypronounced edges are used for the measurement.

An advantageous embodiment of the inventive method is characterized inthat the image spots (n×k) are first measured with n=1, 2, . . . N andk=constant, such as 10, and in that, if no adequately bright image spotwas found, the image spots (n+m)×k are measured with m=1, 2, . . . N,until a sufficiently bright image spot is found. Thereby a search forsuitable image spots is performed efficiently and in the shortest time.

An advantageous embodiment of the inventive method is characterized inthat, for determination of the amplitude value of the image spot, thephase is shifted until the measured amplitude values no longer changesignificantly, and in that the amplitude value then determined isfurther processed.

Alternatively, an advantageous embodiment of the inventive method ischaracterized in that the phase used for determination of the amplitudevalue is advanced sufficiently that the measured amplitude values aresmaller than a predetermined limit value, for example smaller than 50%of the amplitude value, in that the phase is delayed by half the widthof a spot, and in that the amplitude value then measured is furtherprocessed.

The last two of the foregoing embodiments of the inventive method aresimple solutions in order to determine the brightness of the image spotas a prerequisite for determination of the position of the rising andfalling edge of the image spot.

A further advantageous embodiment of the invention is characterized inthat, for determination of the rising edge, the phase is shiftedsufficiently toward the back-porch region that the measured amplitudevalue is reduced to a predetermined percentage, for example 50%, of thepreviously determined amplitude value, and in that this value of thephase is stored temporarily as the position of the rising edge. Yetanother advantageous embodiment of the invention is characterized inthat, for determination of the falling edge, the phase is shiftedsufficiently toward the front-porch region that the measured amplitudevalue is reduced to a predetermined percentage, for example 50%, of thepreviously determined amplitude value, and in that this value of thephase is stored temporarily as the position of the falling edge. In thisway the rising and falling edges of two image spots are determined insimple manner, and the phase can then be adjusted such that it islocated between the rising and falling edges at approximately the centerof an image spot.

A further advantageous embodiment of the invention is characterized inthat the phase or sampling instant is delayed relative to the midpointbetween the rising and falling edges by a predetermined amount, forexample 10% of the width of the image spot. This is advantageous inparticular for rapid video signals with overshoots, since it preventssampling from taking place in the region of the overshoot.

To achieve the object cited hereinabove, the device for matching thephase between the pixel clock of a graphics card and the sampling clockof a flat-panel display having an analog interface in a systemcomprising a flat-panel display, graphics card and computer, ischaracterized by a device that determines the rising edge of a videopulse of a sufficiently bright image spot, a device that determines thefalling edge of the video pulse at a sufficiently bright image spot, andan adjusting device with which the phase is adjusted such that thesampling instant is located at approximately the midpoint between therising and the falling edges of a video pulse.

A further advantageous embodiment of the inventive device ischaracterized by a device which determines the rising edge of a videopulse of a sufficiently bright image spot, a device that determines thefalling edge of the video pulse at a sufficiently bright image spot, andan adjusting device with which the phase is adjusted such that thesampling instant is located at approximately the midpoint between therising and the falling edges of a video pulse.

Further advantageous embodiments of the inventive method and of theinventive device are evident from the remaining dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Practical examples of the invention will be described hereinafter withreference to the attached drawings, wherein:

FIG. 1 shows a block diagram of an analog interface to the graphics cardof a flat-panel display that can be connected to a computer system;

FIG. 2 schematically shows a horizontal synchronization signal and achannel of a video signal, such as the R video signal (R=red color);

FIGS. 3A and 3B show schematic representations of video signals;

FIG. 4 shows a schematic representation of the rising and falling edgesof image spots of a video signal; and

FIGS. 5A and 5B schematically show two ideal video signals and theeffect of the position of the sampling pulse in relation to the videosignal.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a control circuit for a flat-panel display, which can beconnected via an analog interface, and whose function will be explainedin more detail hereinafter with reference to the various input signalsand their conditioning. At the input of the control circuit there areapplied on the one hand the video signal comprising the three colorsignals R, G, B, and on the other hand the two synchronization signalsH-sync and V-sync for horizontal and vertical image synchronization.H-sync and V-sync are transmitted digitally, with signal voltages of 0 Vand >3 V respectively. V-sync signals that the first line of an image isbeing transmitted. This signal therefore corresponds to the imagerefresh frequency and is typically in the range between 60 and 85 Hz.H-sync signals that a new image line is being transmitted.

This signal corresponds to the line frequency and is usually around 60kHz.

The video signal made up of the color signals R, G, B is an analogsignal. The signal voltage ranges from 0 V to 0.7 V. The pixel clock, orin other words the frequency with which the value of this voltage canchange, is 80 MHz. Since a certain number of image spots is transmittedper image line, the pixel clock frequency is higher than the linefrequency (H-sync) by the number of these spots.

The three color signals R, G, of the video signal are fed via a videoamplifier VA to analog-to-digital converters ADCR, ADCG and ADCBrespectively. The two synchronization signals H-sync and V-sync areconditioned in separate circuits HSy, VSy to the effect that the signaledges eroded by transmission and by various EMC processes areregenerated once again. The synchronization signals H-sync and V-syncconditioned in this way are then fed to a microprocessor μP. Thismicroprocessor μP measures their frequency and determines therefrom theresolution adjusted in the graphics card of the computer system. Therespective data stored on resolution are then transmitted to aphase-locked loop PLL and, parallel thereto, to a logic circuit designedin the form of an ASIC for conditioning and processing of the digitaldata.

The phase-locked loop PLL multiplies the frequency of thesynchronization signal H-sync with the value transmitted to it by themicroprocessor μP. Hereby the sampling frequency (pixel clock) isobtained. By virtue of a delay time caused in the phase-locked loop PLL,a phase difference is established between pixel clock and samplingfrequency. These two parameters can be influenced via the OSD displayson the monitor. The sampling frequency obtained in the phase-locked loopis also fed to the three analog-to-digital converters ADCR, ADCG, ADCB.These convert the analog data stream into a digital data stream. Thedigitized data are finally further processed in the downstream logiccircuit ASIC by means of data contained in a video memory VM. Whereas inthe simplest case the data are transmitted in 1:1 correspondence to theflat-panel display that can be connected to the logic circuit ASIC, thevideo memory VM is often used to achieve time decoupling between thearriving data and the data to be transmitted to flat-panel display D.Data stored in video memory VM are also accessed for interpolation oflower resolutions.

FIG. 2 shows the horizontal synchronization signal H-sync and a videosignal of one channel, for example of a red color channel R. The videosignal is selected in such a way in FIG. 2 that bright and dark imagespots are displayed alternately. The broken lines on the video signalshow the ideal sampling instant or the ideal phase for digitization ofthe analog video data. The broken areas on the first two image spotsrepresent the region of the phase which is just still permissible inorder that sampling that is still correct can be achieved. After thephase has been matched, it is therefore located on the broken lines. Ata resolution of, for example, 1024×768 image spots (XGA) and 75 Hz imagerefresh frequency, a fuzzy and highly grainy display is already obtainedat a phase shift of 4 ns. Thus matching of the phase is critical forgood image quality.

From the representations in FIGS. 3A and 3B it can also be seen that thephase of sampling of the video signal plays a large role for imagequality, and that, for different video signals, the phase in many casesmust be located at correspondingly different places. Thus FIG. 3A showsa fast video signal with overshoots, wherein the region of samplingbetween the rising and falling edges of the video signal is relativelynarrow and is shifted toward the falling edge. In contrast, FIG. 3Bshows a slow video signal without overshoots, wherein the region forsampling between the rising edge and the falling edge is relativelybroad and substantially centered. Examination of the two signals showsthat they have phase positions, for example on the right side in theregion of the falling edge of the slow video signal, in which themeasured amplitude values are no longer usable for the slow videosignal, whereas amplitude values that are still usable are measured atthe same phase position of the fast video signal. On the other hand, itis evident that the ideal phase position is located approximately at themidpoint between the rising and falling edges of the video signal andthat it must also be adjusted to this value. Thus it is extremelyimportant to adjust the phase as a function of the respective system.

As already mentioned, automatic phase adjustment is more difficult toachieve than the adjustments of the other parameters. Referring now tothe further figures, it will be described how such an automaticadjustment can be undertaken.

As FIG. 4 shows, the starting point for determination of phase positionis the edges of the video signals. In order to be able to determine anedge, it is advantageous for this to be as pronounced as possible. Thisis the case when the signal is as slightly pronounced as possible aheadof the edge and as strongly pronounced as possible after the edge, orvice versa. The first requirement is ideally satisfied by the samplinggap between the back-porch and front-porch regions, and the second issatisfied by a bright image spot. Accordingly, a bright image spot atthe beginning of a line is highly suitable for determination of therising edge and one at the end of a line is highly suitable fordetermination of the falling edge.

The fact that the edges in question may belong to two different spots,which are possibly located on different image lines, is immaterial,because the pixel clock and sampling clock are known and can be takeninto consideration appropriately. The chosen image spots should havesufficiently high intensity in at least one primary color (RGB) that anedge of sufficiently large amplitude is found.

In principle, any combination of one bright and one dark image spot,which can be located at arbitrary places in the video signal, issuitable for determining the edges. In most cases, the sought edges canbe determined by the combination of front-porch/back-porch region andone bright image spot in the first/last image column. There is then noneed to search through the entire image content for two suitable pairsof spots.

As already illustrated hereinabove, the ideal range for sampling thevideo signal is that in which specified and actual value of the signalare largely in agreement. Measurement of the amplitude of the videosignal in the region of the edge, however, is possible only withdifficulty. The reason lies in the jitter of the video signal and of thesampling pulse. If this is coarse compared with the rise or fall time ofthe video signal, the edges can indeed be found by averaging severalmeasurements, but information on the amplitude of the edge at themeasured place cannot be obtained.

FIGS. 5A and 5B illustrate the problem of detecting the edges. Brokenlines representing the desired sampling instant are inserted at theideal video signals. The hatched area represents the region which, dueto the jitter, is actually sampled in the various measurements. If themeasured values were to be averaged, an average value of about 80% wouldbe obtained in the first case. This averaged value could be incorrectlyinterpreted as a location on the rising edge and, in fact, precisely atthe place at which this has reached 80% amplitude. This is not the case,however. In the second case, the estimate would be 50%, which already iscloser to the true situation.

From these results it is clear that, because of jitter, it will hardlybe possible to determine the place on the edge at which this has reacheda specified value. Under these circumstances, the least error willusually be made by averaging the measured values at about 50% of thespecified value. Obviously other values can also be sought. Smallervalues, for example, have the advantage that less accuracy is necessaryin determination of the actual amplitude of the image spot.

Hereinafter it will be assumed that the image location and the samplingfrequency have already been correctly adjusted. In addition, access tothe data of the analog-to-digital converters will be supposed to bepossible. The rising edge and the falling edge will be determined asfollows, for which purpose the following steps will be performed.

Rising Edge

1. Search for a spot in the first image column which has a sufficientlyhigh, and if at all possible maximal R, G or B value.

2. Since the phase in 1. could have been preadjusted such that themeasurement is erroneous, the actual value of the amplitude may behigher. Determine the actual value of the amplitude by a measurement atsuitable sampling instant by retarding the phase until the measuredamplitude values no longer continue to increase, or by advancing thephase so far at first until the measured amplitude values are very lowand this value of the phase, which marks the beginning of the edge, isstill retarded by half the pixel width.

3. Shift the phase so far toward the back-porch that the sampling valueaveraged over several measurements decreases to about 50% of the valuedetermined in 2. Store this value of the phase temporarily, since therising edge is present here.

Falling Edge

4. Search for one spot in the last image column which has a sufficientlyhigh and if at all possible maximum R, G and B value. In order to obtainthe most accurate possible measured values, the phase should beadjusted, before sampling is performed, to the value found in 2.

5. Shift the phase so far toward the front porch that the averagedsampling value decreases to about 50% of the value determined in 4. Thefalling edge is located at this point.

Alternatively, the sampling instant can also be found by determining therising edge of a video pulse of a sufficiently bright image spot in thefirst image column close to the back-porch region, and by adjusting thephase such that the sampling instant is shifted approximately by halfthe width of an image spot toward the pixel center, or alternatively bydetermining the falling edge of the video pulse at a sufficiently brightimage spot in the last image column close to the back-porch region, andby adjusting the phase such that the sampling instant is shifted byapproximately half the width of an image spot toward the pixel center.Steps 1 to 5 described hereinabove are then correspondingly simplified.

The ideal sampling instant is theoretically located exactly between thetwo edges. In practice, it may be advantageous to sample at a slightdelay from the midpoint between two edges rather than exactly at suchmidpoint, in order to keep away from possible overshoots of the graphicscard and to allow for the often slightly exponential character of theedges.

The hardware of the invention comprises a device which determines therising edge of a video pulse of a sufficiently bright image spot, adevice which determines the falling edge of the video pulse at asufficiently bright image spot, an adjusting device with which the phaseis adjusted such that the sampling instant is located approximately atthe midpoint between the rising and falling edges of a video pulse, anda device for shifting the phase for determination of the sampling valueof the image spot until the measured amplitude values no longer differsignificantly, whereupon the sampling value determined then is furtherprocessed.

Furthermore, a device is provided which advances the phase used fordetermination of the sampling value sufficiently that the measuredamplitude values are smaller than a predetermined limit value, such assmaller than 50% of the sampling value, and by a device which thenretards the phase by half the width of an image spot, whereupon thesampling value measured then is further processed.

Finally, there are provided a device which shifts the phase fordetermination of the rising edge sufficiently far toward the back-porchregion that the measured amplitude value decreases to a predeterminedpercentage, such as 50% of the previously determined amplitude value,whereupon this value of the phase is stored temporarily as the positionof the rising edge, and a device which shifts the phase fordetermination of the falling edge sufficiently far toward thefront-porch region that the measured amplitude value decreases to apredetermined percentage, such as 50% of the previously determinedamplitude value, whereupon this value of the phase is stored temporarilyas the position of the falling edge.

What is claimed is:
 1. A method for matching the phase between the pixelclock of a graphics card and the sampling clock of a flat-panel displaywith an analog interface in a system comprising flat-panel display,graphics card and computer, characterized in that the rising edge of avideo pulse of a sufficiently bright image spot in the first imagecolumn close to the back-porch region is determined, in that the fallingedge of a video pulse of a sufficiently bright image spot in the lastimage column close to the front-porch region is determined and in thatthe phase is adjusted such that the sampling instant is situatedapproximately at the midpoint between the rising and falling edges of avideo pulse.
 2. A method for matching the phase between the pixel clockof a graphics card and the sampling clock of a flat-panel display withan analog interface in a system comprising flat-panel display, graphicscard and computer, characterized in that the rising edge of a videopulse of a sufficiently bright image spot in the first image columnclose to the back-porch region is determined, and in that the phase isadjusted such that the sampling instant is shifted by approximately halfthe width of an image spot toward the center of the pixel.
 3. A methodfor matching the phase between the pixel clock of a graphics card andthe sampling clock of a flat-panel display with an analog interface in asystem comprising flat-panel display, graphics card and computer,characterized in that the falling edge of a video pulse of asufficiently bright image spot in the last image column close to thefront-porch region is determined, and in that the phase is adjusted suchthat the sampling instant is shifted by approximately half the width ofan image spot toward the center of the pixel.
 4. A method according toone of claims 1 to 3, characterized in that the brightness of aplurality of image spots of the first or last image column is measured,and the image spots with the greatest brightness in the first or lastimage column are chosen for determination of the rising or falling edgerespectively of the video pulse.
 5. A method according to one of claims1 to 3, characterized in that the image spots (n×k) are first measuredwith n=1, 2, . . . N and k=constant, such as 10, and in that, if noadequately bright image spot was found, the image spots (n+m)×k aremeasured with m=1, 2, . . . N, until a sufficiently bright image spot isfound.
 6. A method according to one of claims 1 to 3, characterized inthat, for determination of the amplitude value of the image spot, thephase is shifted until the measured amplitude values no longer changesignificantly, and in that the amplitude value then determined isfurther processed.
 7. A method according to one of claims 1 to 3,characterized in that the phase used for determination of the amplitudevalue is advanced sufficiently that the measured amplitude values aresmaller than a predetermined limit value, for example smaller than 50%of the amplitude value, in that the phase is delayed by half the widthof a spot, and in that the amplitude value then measured is furtherprocessed.
 8. A method according to one of claims 1 to 3, characterizedin that, for determination of the rising edge, the phase is shiftedsufficiently toward the back-porch region that the measured amplitudevalue is reduced to a predetermined percentage, for example 50%, of thepreviously determined amplitude value, and in that this value of thephase is stored temporarily as the position of the rising edge.
 9. Amethod according to one of claims 1 to 3, characterized in that, fordetermination of the falling edge, the phase is shifted sufficientlytoward the front-porch region that the measured amplitude value isreduced to a predetermined percentage, for example 50%, of thepreviously determined amplitude value, and in that this value of thephase is stored temporarily as the position of the falling edge.
 10. Amethod according to one of claims 1 to 3, characterized in that thephase or sampling instant is delayed relative to the midpoint betweenthe rising and falling edges by a predetermined amount, for example 10%of the width of the image spot.
 11. A device for matching the phasebetween the pixel clock of a graphics card and the sampling clock of aflat-panel display with an analog interface in a system comprisingflat-panel display, graphics card and computer, characterized by adevice which determines the rising edge of a video pulse of asufficiently bright image spot in the first image column close to theback-porch region, by a device which determines the falling edge of avideo pulse of a sufficiently bright image spot in the last image columnclose to the front-porch region and by an adjusting device with whichthe phase is adjusted such that the sampling instant is situatedapproximately at the midpoint between the rising and falling edges of avideo pulse.
 12. A device for matching the phase between the pixel clockof a graphics card and the sampling clock of a flat-panel display withan analog interface in a system comprising flat-panel display, graphicscard and computer, characterized by a device which determines the risingedge of a video pulse of a sufficiently bright image spot in the firstimage column close to the back-porch region, and by an adjusting devicewith which the phase is adjusted such that the sampling instant isshifted by approximately half the width of an image spot toward thecenter of the pixel.
 13. A device for matching the phase between thepixel clock of a graphics card and the sampling clock of a flat-paneldisplay with an analog interface in a system comprising flat-paneldisplay, graphics card and computer, characterized by a device whichdetermines the falling edge of a video pulse at a sufficiently brightimage spot in the last image column close to the front-porch region, andby an adjusting device with which the phase is adjusted such that thesampling instant is shifted by approximately half the width of an imagespot toward the center of the pixel.
 14. A device according to one ofclaims 11 to 13, characterized by a device for shifting the phase fordetermination of the instant of sampling of the image spot until themeasured amplitude values no longer differ significantly, whereupon thesampling value then determined is further processed.
 15. A deviceaccording to one of claims 11 to 13, characterized by a device whichadvances the phase used for determination of the sampling valuesufficiently that the measured amplitude values are smaller than apredetermined limit value, such as smaller than 50% of the samplingvalue, and by a device which then retards the phase by half the width ofan image spot, whereupon the sampling value measured then is furtherprocessed.
 16. A device according to one of claims 11 to 13,characterized by a device which shifts the phase for determination ofthe rising edge sufficiently far toward the back-porch region that themeasured amplitude value decreases to a predetermined percentage, suchas 50% of the previously determined amplitude value, whereupon thisvalue of the phase is stored temporarily as the position of the risingedge.
 17. A device according to one of claims 11 to 13, characterized bya device which shifts the phase for determination of the falling edgesufficiently far toward the front-porch region that the measuredamplitude value decreases to a predetermined percentage, such as 50% ofthe previously determined amplitude value, whereupon this value of thephase is stored temporarily as the position of the falling edge.